I am implementing a simple Vulkan renderer according to a popular Vulkan tutorial (https://vulkan-tutorial.com/Introduction), and I've run into an interesting issue with the presentation mode and the desktop environment performance.
I wrote the triangle demo on Windows, and it performed well; however, I ported it to my Ubuntu installation (running MATE 1.20.1) and discovered a curious problem with the performance of the entire desktop environment while running it; certain swapchain presentation modes seem to wreak utter havoc with the desktop environment.
When setting up a Vulkan swapchain with presentMode set to VK_PRESENT_MODE_FIFO_KHR and subsequently running the application, the entire desktop environment grinds to a halt whenever any window is dragged. When literally any window on the entire desktop is dragged, the entire desktop environment slows to a crawl, appearing to run at roughly 4-5 fps. However, when I replace the presentMode with VK_PRESENT_MODE_IMMEDIATE_KHR, the desktop environment is immune to this issue and does not suffer the performance issues when dragging windows.
When I researched this before asking here, I saw that several people discovered that they experienced this behavior when their application was delivering frames as fast as possible (not vsync'd), and that properly synchronizing with vsync resolved this stuttering. However, in my case, it's the opposite; when I use VK_PRESENT_MODE_IMMEDIATE_KHR, i.e., not waiting for vsync, the dragging performance is smooth, and when I synchronize with vsync with VK_PRESENT_MODE_FIFO_KHR, it stutters.
VK_PRESENT_MODE_FIFO_RELAXED_KHR produces identical (catastrophic) results as the standard FIFO mode.
I tried using the Compton GPU compositor instead of Compiz; the effect was still there (regardless of what window was being dragged, the desktop still became extremely slow) but was slightly less pronounced than when using Compiz.
I have fully implemented the VkSemaphore-based frame/image/swapchain synchronization scheme as defined in the tutorial, and I verified that while using VK_PRESENT_MODE_FIFO_KHR the application is only rendering frames at the target 60 frames per second. (When using IMMEDIATE, it runs at 7,700 fps.)
Most interestingly, when I measured the frametimes (using glfwGetTime()), during the periods when the window is being dragged, the frametime is extremely short. The screenshot shows this; you can see the extremely short/abnormal frame time when a window is being dragged, and then the "typical" frametime (locked to 60fps) while the window is still.
In addition, only while using VK_PRESENT_MODE_FIFO_KHR, while this extreme performance degradation is being observed, Xorg pegs the CPU to 100% on one core, while the running Vulkan application uses a significant amount of CPU time as well (73%) as shown in the screenshot below. This spike is only observed while dragging windows in the desktop environment, and is not observed at all if VK_PRESENT_MODE_IMMEDIATE_KHR is used.
I am curious if anyone else has experienced this and if there is a known fix for this window behavior.
System info: Ubuntu 18.04, Mate 1.20.1 w/ Compiz, Nvidia proprietary drivers.
Edit: This Reddit thread seems to have a similar description of an issue; the VK_PRESENT_MODE_FIFO_KHR causing extreme desktop performance issues under Nvidia proprietary drivers.
Edit 2: This bug can be easily reproduced using vkcube from vulkan-tools. Compare the desktop performance of vkcube using --present-mode 0 vs --present-mode 2.
